The Sohei was always a fascinating character class option from oriental adventures to me. Inspired by the sohei warrior monks of japan the sohei of oriental adventures drew inspiration from mythology as well as history.
The sohei of history were like the Knights Templar or other similar orders, being a warrior based religious group. They are most recognized as being armed with naginatas (glaives in 5e) parlayance) but in actual history were capable with wakizashi's (scimitar or short sword 5e), daggers, dayikaiu (longbows), kanabo (iron club - 5e maul stats) and wore some form of samurai armour.
So what to do to create a 5e version of this character class. There are a number of ways to go for recreating these warrior monks, fighter variations or even a paladin type class, but IMHO the sohei was not the spell casting type. They were more militant historically than holy, and as such having access to divine magic like a paladin go against the class. However they were meditative and in mythology capable of wuxia type feats of combat prowess.
The 3e oriental version of the sohei had a number of interesting features that can be looked at to create a 5e version.
- must remain lawful
- proficient with all armour (no shields) and with simple and martial weapons
- spells similar to a paladins at 4th level
- ki frenzy - basically a frenzy that gave +2 str, +2 dex and a flurry of blows option
- deflect arrows at 3rd
- resistance to stun and sleep effects 5th
- weapon focus and remain concious feats
- damage reduction at high level
- increased spell resistance to will and fortitude attacking spells
Looking this over there are a number of features that I think that should be kept and a number that could be discarded. The main feature from 3e is the ki frenzy, which is what the entire class is basically built around. So any sohei build IMHO should focus around the barbarian class. However there is a large monk flavour as well so perhaps there can be a blending of the two just re-skinned and a few homebrew rules to give it flavour. The casting options I think can be discarded for a bigger focus on fighting skills.
5e Sohei Class (reflavoured barbarian with some monk spices)
Hit points: as barbarian
Armour: light, medium, heavy, no shields (variant... remove heavy and give unarmoured defence from monk class as a first level option)
Tools: calligraphy supplies (meditative works...)
Saving throws: Strength and Wisdom
Skills: Choose two... Athletics, Acrobatics, Religion, History, Intimidation, Perception
Ki Rage - same as the barbarian rage but reskinned to be due to ki energy
Deflect Missiles- gained at 2nd level instead of reckless attack, use monks deflect missiles concept but as you have no ki points you can't catch or throw them back (hence why its a 2nd level ability at this point)
Danger Sense - re-flavoured to be due to monks innate attunement to their surroundings due to meditative nature
Ability score improvement - same as barbarian
Extra attack - same as barbarian
Fast movement - same as barbarian
Meditative awareness - same as barbarians feral instinct
Ki distruption - same a brutal critical for barbarian
Relentless Ki Frenzy - relentless rage
Persistent Ki Frenzy - persistent rage
Ki Empowerment - indomitable might
Strength of Ki - Primal Champion
One could easily take the Path of the Berserker (Way of the Blurred Blade) with no changes. Path of the Totem Warrior (Way of the Inner Light) however needs some changes to match the Sohei's feel.
Meditative Ritual - replaces spirit seeker, learn the spells Augury (mediate to find the truth) and Purify Food and Drink (tea ceremony) but only as rituals
Shield of the Blade - replaces totem spirit and can only choose eagle with the following variation: while you're raging other creatures have disadvantage on opportunity attack....
Ascetic endurance - replaces aspect of the beast - gain advantage on constitution checks as you learn techniques to slow down your heart and breathing, ignore minor pain or discomfort, control your metabolism consciously etc...
Meditative Insight - replaces spirit walker, learn the commune spell as a ritual, as you mediate on a question that is vexing you.
Leap of the Heavens - replaces totemic attunement- essentially matches the eagle choice from totemic attunement but looks like an amazing leap or run (across spearpoints like in wuxia) while raging.
So there it is. A reskinned barbarian with a strong monk warrior feel. The few changes to the barbarian class take it a little away from a reckless rage monster to a more defined meditative religious warrior, and the changes to the paths make it more divine feeling than nature feeling.